This invention relates to electrodes and combination membrane and electrode assemblies for use with electrochemical cells.
Electrochemical cells are desirable for various applications, particularly when operated as fuel cells. Fuel cells have been proposed for many applications including electrical vehicular power plants to replace internal combustion engines. One fuel cell design uses a solid polymer electrolyte (SPE) membrane or proton exchange membrane (PEM), to provide ion exchange between the anode and cathode. Gaseous and liquid fuels are useable within fuel cells. Examples include hydrogen and methanol, and hydrogen is favored. Hydrogen is supplied to the fuel cell""s anode. Oxygen (as air) is the cell oxidant and is supplied to the cell""s cathode. The electrodes are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. A typical fuel cell is described in U.S. Pat. Nos. 5,272,017 and 5,316,871 (Swathirajan et al.).
Important aspects of a fuel cell include reaction surfaces where electrochemical reactions take place, catalysts which catalyze such reaction, ion conductive media, and mass transport media. The cost of power produced by a fuel cell is in part dependent on the cost of the catalyst. The cost of power produced by a fuel cell is significantly greater than competitive power generation alternatives, partly because of relatively poor utilization of precious metal catalysts in conventional electrodes. However, power produced from hydrogen-based fuel cells is desirable because hydrogen is environmentally acceptable and hydrogen fuel cells are efficient. Therefore, it is desirable to improve the catalyst utilization in fuel cell assemblies to render fuel cells more attractive for power generation.
In one aspect there is provided an electrode structure comprising a current collector sheet, a film comprising a mixture of proton conductive material and carbon particles, the film having a first surface adhered to the current collector sheet, and metallic polycrystals supported on and dispersed on a second surface of the film.
In another aspect there is provided a combination electrolyte and electrode structure for an electrochemical cell comprising a proton-conductive polymer electrolyte membrane and first and second electrodes adhered to opposite surfaces of the membrane. At least one of the electrodes has a layer made up of carbon particles dispersed in a proton conductive material, and metallic polycrystals are dispersed on the layer so as to be facing and at least partially embedded in the membrane. In the preferred embodiment, the carbon particles have a mean particle size in the range of about 35 to about 50 nanometers, and the metallic polycrystals are platinum. The electrolyte membrane and the proton conductive material preferably each comprise a copolymer of tetrafluoroethylene and perfluorinated monomers containing sulfonic acid groups.
In one embodiment there is provided a method of making the improved electrode structure described above for use in an electrochemical cell. The electrode is produced by forming a mixture comprising proton-conductive material and carbon particles, applying the mixture to a current collector sheet to form a film, and dispersing a catalyst in the form of metallic polycrystals on the exposed surface of the film. This method produces an electrode having significantly increased catalyst utilization, dramatic reduction of catalyst loading, and which is consequently less expensive to produce than electrodes produced by prior art methods.
In a preferred embodiment, the film is preferably prepared by mixing the proton-conductive material and carbon particles with a solvent, spreading the mixture on the current collector sheet, and subsequently evaporating the solvent. The polycrystals are then deposited on the film by a physical vapor deposition process such as electron beam evaporation. The physical vapor deposition process allows the catalyst to be deposited on the electrode film without subjecting the film to high temperatures which would degrade or destroy the proton-conductive material. The end result is the catalyst localized in an ultra-thin layer in intimate contact with the film. The resulting film is preferably hot-pressed onto the current collector sheet in order to insure proper adhesion.
There is also provided a method of making a combination electrolyte and electrode structure for an electrochemical cell having an electrolyte membrane of solid polymer proton-conductive material and first and second electrodes disposed on either side of the electrolyte membrane, at least one of the electrodes being formed by applying a mixture comprising proton-conductive material and carbon particles onto a current collector sheet to form a film which adheres to the sheet, and forming dispersed metallic polycrystals on the surface of the film. The electrode produced in this method is then placed on a first surface of the electrolyte membrane such that the metallic polycrystals face the membrane. The second electrode is placed on the opposite surface of the membrane and the resulting structure is heated and compressed to adhere the electrodes to the membrane. In a preferred embodiment of the invention method the electrodes are adhered to the membrane by subjecting the assembly to a compressive load of from about 250 to about 1000 pounds per square inch, and a temperature of from about 280xc2x0 F. to about 320xc2x0 F., and maintaining these conditions for about 1 to about 5 minutes. These conditions have been found to result in the metallic polycrystals becoming at least partially embedded in the membrane, thereby providing a continuous path for protons to the catalyst site where reaction occurs.
As can be seen from the description of the electrode, membrane electrode assembly, and the fuel cell system described above, the invention provides improved catalyst utilization and reduced catalyst loading.
It is an object of the invention to provide new electrodes and new membrane electrode assemblies. Another object is to provide a method for preparing the electrodes and assemblies containing the improved electrodes. Advantageously the membrane/electrode assembly of the invention provides relatively high power output with unexpectedly low catalyst loading.
These and other objects, features and advantages will become apparent from the following description of the preferred embodiments, claims, and accompanying drawings.